Disconnectable turret mooring systems are known from the prior art, for instance from WO2007/077126.
Disconnectable turret mooring systems comprising a mooring buoy and a turret structure. The mooring buoy is anchored to the seabed with anchoring legs. The turret structure, provided on a vessel, has a receptacle for receiving the buoy member and one or more buoy locking devices for locking the buoy member in the receptacle.
The turret structure may be an internal turret structure or an external turret structure. An internal turret structure is provided inside the hull of the vessel, in a so-called moonpool of the vessel. The receptacle is formed as an opening at or near the bottom of the vessel, facing downwards. An external turret structure is provided outside the hull of the vessel. The external turret is fixed with suitable connection members to the bow or stern of the vessel.
The mooring buoy may be moved up and down, i.e. from a temporary storage position at a safe distance below the water surface (e.g. 30-200 meters) to a mooring position close to or at the surface of the water where it can be received by the receptacle and connected to the vessel.
The turret structure itself is connected to the vessel, but is rotatably held within the moonpool of the vessel, allowing the vessel to weathervane under influence of wind, waves and currents. The buoy mooring system may be disconnected and reconnected to the turret structure when needed, thereby providing a disconnectable turret mooring system.
The turret mooring system comprises a fluid transfer system to allow transportation of hydrocarbon fluids, for instance by establishing a flow path between the vessel and a subsea well via the turret structure and the mooring buoy.
The turret structure may comprise a first part of the fluid transfer system and the mooring buoy may comprise a second part of the fluid transfer system. The turret structure comprises fluid paths and a turret manifold and the mooring buoy comprises fluid paths as well which are connected to the risers. The turret structure and mooring buoy fluid paths are matching; conduits of the turret manifold can be connected to corresponding conduits of the buoy to establish one or more hydrocarbon fluid flow path.
During the connection of the mooring buoy to the turret structure, it is important to align both parts of the fluid transfer system to allow proper connection of the fluid transfer system.
This may be done by rotating the turret structure with respect to the vessel before connecting to the mooring buoy. According to state of the art in disconnectable turret mooring systems, the alignment has to be done during a critical phase just before the locking of the mooring buoy into the receptacle of the turret structure.
WO2007/077126 shows an alternative solution. This solution is to connect the mooring buoy without any specific attention as to its orientation. Only after the vessel has safely been connected to the mooring buoy, a turntable which is supported on the turret structure and which is rotating with respect to the turret structure, is rotated to align the turret manifold conduits with the risers that are connected to the buoy when the buoy member is received and locked in the receptacle of the turret structure.
According to another application WO2009/141351 only after the vessel has safely been moored via its turret to the mooring buoy, a turntable, which is supporting the upper turret manifold decks and swivel stacks, is supported with hydraulics which are provided to lift the turntable a few mm so to allow its rotation. Furthermore, a special turntable bearing system and a turntable motor drive system are provided to rotate the turntable in order to align the turret manifold to the buoy manifold.
After alignment, the turntable can be lowered (by a few mm) onto the turret by deactivating the hydraulics. The rotated and aligned turntable and turret structure can be locked together and secured in that position.
The fact that the complete turret manifold can be orientated with regard to the turret structure and the mooring buoy after connection, avoids having to perform the alignment of the turret manifold with respect to the buoy manifold during the critical stage of connecting the buoy member to the turret structure.
However, in these systems, it is requested to operate a heavy mass that generates very high friction torques during rotation and requires a relatively strong, expensive and inaccurate mechanical aligning device.
Different embodiments using turntables are described in US2007155259, U.S. Pat. No. 5,651,708. However, these known mooring systems are relatively complicated and vulnerable for malfunction. For instance, according to WO2009/141351, heavy and complex hydraulics for lifting the turntable, are required.
It is therefore an object of the present invention to provide a turret mooring system that avoids the need of having to perform the alignment of the turret manifold with respect to the buoy manifold during the critical stage of connecting the buoy member to the turret structure, while at the same time, it is not required to provide lifting means that are capable of lifting the full weight of the turret manifold including the structure, the piping, the mechanical equipment, prior to the orientation.